This invention generally relates to methods for decontaminating the primary side of a nuclear steam generator by the removal of radioactive debris from fuel assemblies and is specifically concerned with the removal of such debris by immersing such fuel assemblies in water and subjecting them to shock waves generated by underwater pulses of pressurized gas.
Methods and systems for decontaminating the primary side of nuclear steam generators are known in the prior art. However, before the purposes and operation of these methods can be appreciated, some understanding of the structure and operation of a nuclear steam generator is necessary.
Nuclear steam generators are comprised of three principle components, including a secondary side, a tube sheet, and a primary side which circulates water heated from a nuclear reactor. The tube sheet hydraulically isolates the primary side from the secondary side. The secondary side of the generator includes a plurality of U-shaped heat exchanger tubes, as well as an inlet for admitting a flow of water. The inlet and outlet ends of the U-shaped tubes within the secondary side are mounted in the tube sheet that hydraulically separates the primary and the secondary sides. The primary side in turn includes a divider sheet which hydraulically isolates the inlet ends from the U-shaped tubes from the outlet ends. Hot, radioactive water flowing out of the core of the nuclear reactor is admitted into the section of the primary side containing all of the inlet ends of the U-shaped tubes. This hot, radioactive water flows through these inlets, up through the tube sheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. This water from the reactor transfers its heat through the walls of the U-shaped tubes to the non-radioactive feed water flowing through the secondary side of the generator, thereby converting feed water to non-radioactive steam that in turn powers the turbines of an electric generator. After the water from the reactor circulates through the U-shape tubes, it flows back through the tube sheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is recirculated back to the nuclear reactor.
The primary side of the generator includes a core barrel which houses approximately one hundred nuclear fuel assemblies which are uniformly spaced from one another. Each of the fuel assemblies, in turn, comprises a rectangular array of approximately two hundred fuel rods which are supported and contained within the fuel assembly skeleton. The skeleton, in turn, is formed from seven grids which are uniformly connected along an array of thimble rods. Each of the grids is formed from two sets of parallel, metal plates orthogonally disposed with respect to one another and which interfit in "egg crate" fashion to define a pattern of square cells which receive, support and uniformly space the fuel rods from one another. The walls of each of the cells in turn include spring fingers for resiliently biasing the center line of the rod along the center line of the cell.
In operation, the fuel rods may attain a temperature of 1800 degrees F. along their center lines as the result of the fission reaction which occurs within them. The heat generated by this fission reaction is removed by water which circulates between the reactor core and the inlet and outlets of the U-shaped heat exchanger tubes in the secondary side of the generator. Over time, radioactive debris is generated within the primary side both by the corrosion of the zircaloy cladding of the fuel rods, and the stainless steel components of the fuel assembly skeleton, as well as by the reduction of solid material out of the water from the nucleate boiling which occurs at the 1800 degree F. surface of the fuel rods.
As a result of the exposure of this debris to the intense radioactivity generated by the fuel rods, this debris becomes highly radioactive. Moreover, over time, particles of debris will break off of the fuel assemblies and become entrained in the water circulating through the primary side. Unfortunately, these highly radioactive particles of debris do not circulate continuously through the piping of the primary side; instead, they tend to deposit themselves at points along the primary side which generate regions of non-uniform flow, such as valves and elbow joints. The end result of this deposition over time is that the valves and elbow joints in the piping of the primary side can become radioactive enough to pose a very real radiation hazard to the maintenance operators which routinely service the piping in the containment area of the nuclear facility.
Various techniques for removing this highly radioactive particulate debris from the primary side have been developed in the prior art. Such techniques include the introduction of caustic chemicals in the primary side which dissolve and remove such contaminates, as well as the scrubbing of the channel head regions of the primary side by a high pressure stream of a water-grit mixture which abraids and rinses these particulate contaminates away (see for example U.S. Pat. Nos. 4,226,640 and 4,374,462). Unfortunately, the use of caustic chemicals may corrode and thin out the walls of various pipes and tubing in the primary side, while the use of a water-grit "sand blast" has been found to be effective only in localized areas of the primary side. Moreover, all of the techniques used to date have proven to be extremely expensive to implement.
Clearly, there is a need for a method for effectively decontaminating the primary side of a nuclear steam generator which is both effective throughout all portions of the primary side and relatively inexpensive.